The field of photochromic glasses is founded in U.S. Pat. No. 3,208,860 which discloses the production of silicate-based glasses that exhibit darkening when exposed to actinic radiation, customarily ultraviolet radiation, and which return to their original color when removed from the source of actinic radiation. Such reversible optical properties are achieved via the incorporation of effective amounts of silver and at least one halide of the group chloride, bromide, and iodide into the glass composition which combine to form silver halide crystallites in the glass. The crystallites are so small as to be invisible to the unaided eye, yet are darkenable under the action of actinic radiation to reduce the optical transmittance of the glass. When the actinic radiation is extinguished, the crystallites fade to their original state, thereby restoring the optical transmittance to its initial level. This cycle of darkening and fading can be repeated indefinitely without fatigue in photochromic glasses.
By far the most prevalent use for photochromic glasses has been in the fabrication of ophthalmic lenses. One example of that application is provided in U.S. Pat. No. 3,197,296 which describes a family of refractive index-corrected silicate glasses containing silver halide crystals to provide the desired photochromic behavior. Those glasses demonstrated, in conventional 2 mm thickness, photochromic properties sufficiently developed for prescription ophthalmic applications along with the necessary refractive index to be compatible with conventional lens grinding practices.
The manufacture of ophthalmic lenses commonly involves the pressing of glass lens blanks of optical quality from a melt followed by the grinding and polishing of the blanks to predetermined prescriptions. It is believed self-evident that the production of non-prescription photochromic glass lenses, for example, sunglass lenses, in large quantities by processes demanding grinding and polishing is not only expensive and time consuming, but is also wasteful of material. Consequently, less costly means for producing photochromic glass sheet for lenses or other applications would be highly desirable. Assuming that the sheet could be produced in optical quality, the sheet could be inexpensively thermally sagged to the curvatures required for lenses, windshields, and other sheet glass configurations.
The commercial sheet glass forming processes practiced today contemplate maintaining substantial volumes of molten glass at temperatures wherein the glass has the necessary viscosity for sheet forming, viz., at a viscosity between about 10.sup.4 -10.sup.6 poises. By the very nature of the drawing process, those volumes of glass are in prolonged contact with refractory metals or ceramics which serve as the means for forming drawn sheet. Thus, the sheet drawing processes impose severe constraints upon glass composition because of the formidable liquidus and glass stability problems associated with the handling and processing of glass at relatively low temperatures and high viscosities.
In addition to good formability properties, suitable glass sheet for ophthalmic purposes will exhibit high optical quality, good chemical durability, high strength, and good photochromic darkening even in sheet of moderate thickness. Where the sheet is scheduled for use as lightweight sunglass lenses, the glass must also be chemically strengthened such as to meet the Food and Drug Administration (FDA) requirements for eyeglass lens safety. Federal safety requirements cannot be routinely met in lightweight glass of moderate thicknesses (1.3-1.7 millimeters) in the absence of chemical strengthening, or by utilizing an air tempering procedure. U.S. Pat. No. 4,018,965 describes a group of glass compositions which demonstrates the properties necessary for photochromic sheet glass applications.
From the considerable experience gained through the years in the manufacture of photochromic glasses suitable for ophthalmic applications, the following several criteria have been formulated therefor as goals to achieve in the production of sunglasses; these criteria being in addition to the necessary melting, forming, and chemical strengthening capabilities, as well as the physical characteristics conventionally demanded in non-photochromic ophthalmic ware.
First, a glass which in 1.5 mm thickness at room temperatures (25.degree.-30.degree. C.) will exhibit an optical transmittance in the range of 60-90% before exposure to actinic radiation but which, when irradiated with actinic radiation, e.g., bright outdoor sunlight, will darken to a transmittance of less than 30%.
Second, a glass which in 1.5 mm thickness at 25.degree.-30.degree. C. will fade very rapidly when removed from the incident actinic radiation; i.e., the glass within five minutes will fade to a transmittance of about two times its darkened transmittance and within an hour will fade to a transmittance of at least 80% of its original transmittance.
One circumstance which must be kept in mind when conducting research involving photochromic glass is the fact that the dynamics of photochromic behavior exhibited by glasses are directly related to the intensity of the actinic radiation impinging thereon and the temperature of the glass while being irradiated. Accordingly, where other parameters are held constant, a photochromic glass will customarily darken to a lower transmittance when exposed to actinic radiation while at a lower temperature. Moreover, the intensity of solar radiation can obviously vary greatly depending upon the season of the year, the location of the exposure (angle of declination of the sun), cloud cover, snow cover, air mass value, etc.
With respect to temperature dependence, i.e., the degree of darkening demonstrated by a photochromic glass over a range of ambient temperatures, some photochromic glasses in 1.5 mm thickness may darken to a transmittance of less than 5% when subjected to solar radiation at a temperature of -18.degree. C. (0.degree. F.). Such glasses would not comply with the specifications of the American National Standards Institute (ANSI) which specify lenses for general use as fixed tint sunglasses to exhibit an optical transmittance of at least 5%.
Consequently, a third criterion proposed for photochromic glasses which are to be used for ophthalmic applications is that in 1.5 mm thickness the glass will not darken to a transmittance of less than 5% at -18.degree. C.
The converse of the above-stated rule regarding temperature dependence also holds true; viz., where other parameters are maintained constant, a photochromic glass will darken to a lesser degree, i.e., the final darkened optical transmittance will be higher, when the glass is at a higher temperature when exposed to actinic radiation. To have practical utility as a sunglass, it has been deemed that a photochromic glass should darken to an optical transmittance of less than 50% when exposed to outdoor sunlight at temperatures encountered during summer.
Accordingly, a fourth criterion which has been proposed is that a photochromic glass in 1.5 mm thickness will darken to a transmittance less than 50% when exposed to actinic radiation at 40.degree. C. (104.degree. F.).
Finally, to simplify manufacturing techniques, while concomitantly maintaining the optical properties of the pristine glass surface, the ideal glass compositions would permit the desired photochromic properties to be developed concurrently with the required lens curvature during a thermal sagging operation. U.S. application Ser. No. 773,958, filed Mar. 3, 1977 in the names of Bourg, Hazart, and Jouret now U.S. Pat. No. 4,088,470, discloses such a technique for simultaneously heat treating and sagging sheet of photochromic glass into lens blanks of a desired curvature.
It is believed apparent from the prior art that the photochromic properties exhibited by a particular glass are dependent upon both composition and the heat treatment to which it is subjected. The curvature secured in a thermal sag cycle is also a function of such parameters as glass composition and incident thermal cycle resulting through the combined effects of surface energy, density, and viscosity, this latter factor being strongly dependent upon temperature. A most fortuitous circumstance would exist where the desired photochromic behavior could be achieved through the same heat treatment as that giving rise to the necessary lens curvature.
Therefore, a fifth criterion proposed is a glass capable of being concurrently heat treated and sagged to simultaneously yield the desired lens curvature and photochromic properties.
U.S. application Ser. No. 887,677, filed Mar. 17, 1978 by G. B. Hares, D. L. Morse, D. W. Smith, and T. P. Seward, III, now abandoned and refiled Feb. 28, 1979 as U.S. application Ser. No. 14,981, now U.S. Pat. No. 4,190,451 discloses a silver halide-containing, silicate photochromic glasses exhibiting quite rapid fading characteristics and relatively low temperature dependence of darkening. Several of the compositions recited in that application are operable for sheet drawing processes but are not suitable for a simultaneous heat treating-sagging procedure, such as has been described above. The inapplicability of those glasses for such a process resides in the fact that the times and temperatures demanded to sag the glass sheet are such as to cause the glass to sag into contact with formers which produce the necessary lens curvature, this contact causing the destruction of the good optical properties of the pristine surface. Yet, without such formers, those glasses would sag to a much higher curvature than desired. Thus, the lens blanks fabricated from those glass compositions via a heat treating-sagging technique would require grinding and polishing to provide the required optical quality surface.